In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are often utilized to dope a workpiece, such as a semiconductor wafer, with ions from an ion beam, in order to either produce n- or p-type material doping, or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductor wafers, the ion implantation system injects a selected ion species into the workpiece to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic, or phosphorus, for example, results in an “n-type” extrinsic material wafer, whereas a “p-type” extrinsic material wafer often results from ions generated with source materials such as boron, gallium, or indium.
Typical ion implantation systems include an ion source for generating electrically charged ions from ionizable source materials. The generated ions are formed into a high speed beam with the help of a strong electric field and are directed along a predetermined beam path to an implantation end station. The ion implanter may include beam forming and shaping structures extending between the ion source and the end station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the ion beam passes en route to the end station. During operation, this passageway is typically evacuated in order to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
It is common for the workpiece being implanted in the ion implantation system to be a semiconductor wafer having a size much larger than the size of ion beam. In most ion implantation applications, the goal of the implantation is to deliver a precisely-controlled amount of a dopant uniformly over the entire area of the surface of the workpiece or wafer. In order to achieve the uniformity of doping utilizing an ion beam having a size significantly smaller than the workpiece area, a widely used technology is a so-called hybrid scan system, where a small-sized ion beam is swept or scanned back and forth rapidly in one direction, and the workpiece is mechanically moved along the orthogonal direction of the scanned ion beam. A special form of a hybrid scan, for example, is a so-called DC ribbon beam scan system, in which the scan frequency is theoretically raised infinitely high in order to form a generally continuous DC beam.
In a typical hybrid scan ion implanter, scan frequencies in the two orthogonal directions are quite different, in a way that is similar to the raster scan on a conventional CRT television screen. In the mechanical motion direction of a hybrid scan ion implanter, for example, the scanning motion repeats in a sub-Hz frequency, typically 0.5 Hz as a maximum repetition rate, whereas in the ion beam scan direction, the scanning frequency is as high as 1 KHz.
One of the attractions of a hybrid scan system is that the final doping uniformity on the workpiece is controlled independently in the two orthogonal directions. Once beam current density in the beam scan direction is made uniform, simply moving the workpiece at constant speed in the orthogonal direction will provide the desired 2-dimensional, or 2-D, uniformity. Even if ion beam current fluctuates during the workpiece motion, the speed of the workpiece can be varied accordingly to yield the desired uniformity, assuming the uniformity in the beam scan direction stays in an acceptable range.
Non-uniformity in the beam scan direction results from a non-linearity of the beam scan and optics associated the ion beam formation mechanism. An analogy is a paint spray can that is located at a fixed point from a wall, where the spray can pivots left to right at a constant angular velocity. At constant angular velocity, wall areas far from the spray can receive thinner paint coverage than the area in directly in front of the spray can. In this analogy, the changing distance from the spray can to the wall area is a source of non-linearity in paint coverage.
To obtain uniform beam distribution in the ion beam scan direction of an ion implantation system, the degree of uniformity has to be first measured, and then appropriate correction is made to correct non-uniformities. In the spray paint analogy, once the non-uniformity of the paint is known, angular velocity of spray can is modified so that the spray can slows down as it sprays farther from the center.
However, since the goal of the uniformity in an ion implantation is less than one percent non-uniformity, there are many difficult aspects in attaining uniformity. One difficulty is reliably measuring the uniformity in the beam scan direction under the presence of beam current fluctuations in time. Since measuring beam uniformity is intended to establish a time-independent variation of beam intensity along the beam position, beam fluctuations in time pose substantial difficulties.